The objective of the proposed research is to elucidate the mechanism of phototransduction using single-step mutants of Drosophila. To date, 83 mutations have been isolated from 12 different genes, all of which seem to affect the phototransduction process. Physiological and biochemical studies will be carried out on these mutants and double mutants constructed in pair-wise combinations of some of the key mutations. From the results of these studies we hope to obtain clues to the actions of each gene in terms of physiological and biochemical parameters and to the sequence of gene actions and interaction among the gene products. Some of the mutations will be tested to see if they are lesions in the structural gene(s) for opsin. The structural gene for opsin, if found, will be used to probe the functional roles of rhodopsin in transduction. Techniques for recording from isolated photoreceptors and for introducing exogenous substances into the isolated preparation will be developed to test the effects on the mutant photoreceptors of the substances that are found to be, or suspected to be, lacking in the mutants.